Outbreaks of red tide along the South African
coast are reported periodically in the media, with
warnings to the public about the dangers of
collecting and consuming shellfish in the affected
areas. However, few people understand how a red tide
develops, or how they might be affected by this
phenomenon.

WHAT
IS A RED TIDE?

The term red tide is misleading, because
discolourations of the water may be brown, orange,
purple or yellow, as well as red. These
discolourations are caused by dense concentrations of
the microscopic plants of the sea, the so-called
phytoplankton. The discolouration varies with the
species of phytoplankton, its pigments, size and
concentration, the time of day and the angle of the
sun.
Most red tides along the South African coast are
caused by a group of phytoplankton known as
dinoflagellates. These single-celled organisms are
able to swim short distances by means of two
whip-like appendages called flagella. Their ability
to swim contributes to their success as red tide
organisms, for reasons that will be explained later.

WHAT
TRIGGERS A RED TIDE?

Red tides usually occur along the Cape west or
south coasts in late summer and autumn. The
prevailing southerly winds at this time of year cause
cold, nutrient-rich water to rise up from the deeper
regions of the ocean to the surface, a process known
as upwelling. Swept along with this upwelled water
are dinoflagellate cysts, the resting stages of the
organism, which lie dormant in the sediments on the
sea floor. The high nutrient concentrations in the
upwelled water, together with ideal conditions of
temperature, salinity and light, trigger the
germination of the cysts, so that the dinoflagellates
begin to grow and divide. The rapid increase in
dinoflagellate numbers, sometimes to millions of
cells per litre of water, is what is known as a
"bloom" of phytoplankton. Concentration of
the bloom by wind and currents, as well as the
dinoflagellates' ability to swim to the surface,
together lead to the formation of a red tide.
Red tides are therefore not a result of sudden
population explosions of phytoplankton attributable
to increased division and growth rates, but result
from confinement or concentration of phytoplankton
following normal population increases to such an
extent that they discolour the water.
If conditions in the surface waters become
unfavourable for the dinoflagellates, for example if
the nutrients are depleted or the bloom is dispersed
by wind and currents, the dinoflagellates will again
form dormant cysts and sink to the sea floor.

WHAT
ARE THE CONSEQUENCES OF RED TIDE?

Most red tides represent useful contributions to
plankton production but some periodically produce
harmful results.

PHYSICAL
DAMAGE

Dense concentrations of red tide organisms can
suffocate fish by clogging or irritating their gills,
so that they cannot extract sufficient oxygen from
the water. During 1962 the mortality of more than 100
tons of fish in False Bay was
attributed to gill clogging by the dinoflagellate Gonyaulax
polygramma. Other forms of physical damage
include the recently discovered feeding on fish
tissue by certain dinoflagellate species resulting in
the death of the fish within a matter of hours. Such
events are now thought to have been responsible for
many unexplained fish kills in the past.

OXYGEN
DEPLETION

Red tides may also kill indirectly by depleting
the oxygen dissolved in the water. The mass mortality
of the red tide organisms once the nutrients have
been depleted results in an increase in the number of
bacteria which are responsible for decomposition in
the sea. The activities of this huge population of
bacteria soon deplete the oxygen concentration in the
water, leading to the
death of other marine animals. Low oxygen levels
following such blooms are believed on a number of
occasions to have caused rock lobsters
to crawl from the sea. Such an event was observed in
St Helena Bay in 1978 following a bloom of the
photosynthetic ciliate Mesodinium rubrum.

* In March 1994, South Africa experienced its worst recorded marine mortality in the west coast area of St. Helena Bay. The event was caused by the entrapment and subsequent decay of an extensive red tide dominated by the non-toxic Prorocentrum micans and Ceratium furca, with the toxic species Alexandrium catanella and Dinophysis acuminata present in lower concentrations. Marine life died because of suffocation or hydrogen sulphide poisoning. The low oxygen conditions allowed anaerobic,
sulphate-reducing bacteria to convert sulphates in the water column to hydrogen sulphide gas, which corroded metal objects and caused respiratory problems amongst some residents of the area. These chemical reactions also caused the sea to turn black, and the event was soon dubbed a black tide by the media. Approximately 60 tons of crayfish and 1500 tons of fish, comprising about 50 species, washed ashore. The mullet, Liza richardsnii, made up the bulk of the fish mortality (1250 tons) with the remainder being dominated by sharks, and bottom-dwelling fish. Along the bays rocky shores, most of the mussels, limpets, sea urchins and other intertidal life had died, with the exception of the false limpet, Siphonaria capensis, which is capable of switching to anaerobic metabolism in the absence of oxygen.

DIRECT
POISONING

Toxins produced by certain dinoflagellates are
some of the most potent poisons known to man. The
most notorious of the dinoflagellate toxins are the
neurotoxins which disrupt normal nerve functions.
Toxins of this nature have caused
numerous marine mortalities on the South African
coast. Virtually the entire adult mussel population
in the Elands Bay area was destroyed by the
dinoflagellate Gonyaulax catenella in 1980,
while 30 tons of abalone
were washed up in the HF Verwoerd
Marine Reserve in 1989, following blooms of the
dinoflagellate Gymnodinium nagasakiense, a
recognized fish killer in the coastal waters of
Japan.

INDIRECT
POISONING

Animals such as mussels, clams and oysters are
particularly vulnerable to red tides, because they
feed by filtering particles, including phytoplankton,
from the water. Toxic phytoplankton accumulate in the
digestive system of these filter-feeders and
subsequently cause illness or death to consumers such
as birds, marine mammals and man.
This then, is the category of red tide about which
the public is periodically warned. It should be
remembered that cooking only slightly lessens the
toxicity of affected shellfish because the toxins are
generally heat stable - in other words, they are not
destroyed by heat. However, most cases of poisoning
are restricted to filter-feeding shellfish, and other
seafood may be consumed safely.

Four different types of indirect poisoning have
been identified as harmful to man.

Paralytic
shellfish poisoning (PSP)

PSP was discovered in the 1700s and is in many
respects the most serious of the shellfish
poisonings: several hundred human deaths have been
recorded worldwide during the past 300 years. Along
the South African coast the dinoflagellate
Gonyaulax catenella is regularly responsible
for PSP on the West Coast and has caused several
human deaths.

A number of toxins are responsible for PSP. The
most common is saxitoxin, which disrupts normal nerve
functions. It is extremely potent and may become so
concentrated that consumption of a single mussel can
be fatal. The first symptoms are a tingling,
prickling, stinging or burning sensation of the lips,
tongue and fingertips within 30 minutes of eating
poisonous mussels. Numbness
of the arms, legs and neck follows. Other symptoms
develop later and include dizziness, general muscle
incoordination, headaches, vomiting and impaired
respiration. Death is by respiratory failure and may
occur within 2 - 24 hours.
Mussels may remain toxic for some time after the
occurrence of this type of red tide. If the red tide
disappears completely the mussels may take only a few
weeks to flush the toxins from their systems.
However, if the red tide organisms remain in the
water at low concentrations the mussels may remain
toxic for several months.

Diarrhetic
Shellfish Poisoning (DSP)

DSP has only recently been recorded for the first
time on the South African coast.
The causative organism has been identified as the
dinoflagellate Dinophysis acuminata, which
produces the toxin okadaic acid. The symptoms, which
usually occur within four hours but may persist for
three days, include diarrhoea, nausea, vomiting,
stomach ache and shivering. It is likely that DSP has
gone unreported on many occasions because of the
relatively mild nature of the symptoms. In addition,
the symptoms may be confused with those of
gastro-enteritis associated with the consumption of
shellfish from polluted waters.

Neurotoxic
Shellfish Poisoning (NSP)

Along the South African coast the dinoflagellate Gymnodinium
nagasakiense is usually implicated in NSP. Most
outbreaks have been reported from False Bay, where
they are responsible for the olive-green
discolouration of the seawater during autumn.
The symptoms of NSP are sensory abnormalities and
include dizziness, tingling sensations, dilated
pupils and hot-cold reversals. These symptoms usually
disappear in three days and no human deaths have yet
been documented. Red tides of this type may also
cause irritations of the human eye, nose and throat
through contact with sea spray.
As a result of the mass mortalities of marine animals
associated with these red tides, the public should
exercise caution in the collection of seafood.

Amnesic
Shellfish Poisoning (ASP)

ASP was recorded for the first time off the coast
of Canada in 1987 when three deaths and over 100
confirmed cases of acute intoxications followed the consumption of cultured mussels. Nitzschia
pungens, a species of phytoplankton belonging to
a group known as the diatoms, was identified as the
causative organism, producing the neurotoxin domoic
acid. Symptoms of ASP include abdominal cramps,
vomiting and neurological responses involving
disorientation and memory loss. Although ASP has not
been recorded off the South African coast, the
responsible organism is thought to occur in our
waters.

* Aerosol (air-born) Toxins

In the summer of 1995-96, South Africa experienced a severe aerosol toxin problem in False Bay which later spread to the coastal resort of Hermanus in Walker Bay. Beachgoers and seaside residents were overcome by the discomfort of coughing, burning of the nasal passages, difficulty in breathing, stinging eyes and irritation to the skin. Although the discomforts experienced were considerable, symptoms were usually relieved upon leaving the area, and no long-term effects were noted. The aerosol toxin was linked to the presence of the bloom of a toxic dinoflagellate species Gymnodinium, first recorded in False Bay in 1988. This species appears to represent a new species of southern hemisphere Gymnodinium. Despite the species having bloomed on several occasions since then, never before have the noxious effects to humans been so evident as during the 1995-96 event. Faunal mortalities were however small, with the exception of the larval mortalities experienced by several land-based abalone farmers in the Walker Bay area. The presence of aerosol toxins which result in respiratory distress in human has thus far only been recorded from the west coast of Florida (for many years now) USA, and in 1993 from New Zealand.

RED
TIDE AND THE ROLE OF THE SEA FISHERIES RESEARCH
INSTITUTE

In recent years there has been a growing awareness
worldwide of the problems associated with red tides,
largely due to the expansion of the shellfish
industry and the increased risk to human health. In
addition, scientists have concluded that red tides
are occurring with increasing intensity and frequency
over a wider global distribution, and that this may
be a result of human activities. Nutrient enrichment
through various forms of pollution, and subtle
changes ascribed to the greenhouse effect, are
thought to have influenced the intensity and
frequency of red tides. Also, the transport of
dormant cycts in the ballast tanks of ships is
thought to have contributed to the spreading
distribution of red tide outbreaks. It has therefore
been recommended that an international research
effort be undertaken to evaluate the global expansion
of algal blooms and man's involvement in it.
The aim of this research includes finding a way of
accurately predicting red tide outbreaks, so ensuring
that steps can be taken to warn the public in
advance. Simple predictive models have been developed
for certain dinoflagellate species, but these can
only be applied to specific areas. Good monitoring
programmes are therefore still the most satisfactory
means of providing an efficient warning system.
In South Africa the responsibility for monitoring red
tide rests with the Chief Directorate Sea Fisheries
of the Department of Environment Affairs. Researchers
from the Sea Fisheries Research Institute (SFRI)
regularly monitor the waters around our coast for red
tide outbreaks in order to warn the public of
potentially harmful blooms. Members of the public
should notify the SFRI at 021-4396160 of unusual
discolourations of the sea or illness following the
consumption of shellfish. The Red Tide
Hotline (021-4394380) is a 24-hour answering service
that provides information concerning outbreaks of red
tide detected by the SFRI.

ACKNOWLEDGEMENTS

The original text was written by Irma van der
Vyver and Dr Grant Pitcher. Additional information courtesy of Sue Matthews* and edited by Gavin W. Maneveldt. Photographs by Deon
Horstman and Dr Grant Pitcher. We kindly thank Dr.
Pitcher for permission to reproduce his booklet for
the Web.